The “valve metal” as referred to herein means a metal having a so-called valve action (rectification action) in which an oxide layer on the subject metal passes a current therethrough only in one direction but does not substantially pass a current therethrough in a reverse direction thereto (Handook of Metal Finishing Technology (Revised New Edition), page 712 (1976), edited by The Metal Finishing Society of Japan); and the oxide film formed on a surface of a material to be treated containing a valve metal as a principal component differs from oxide films formed on other noble metal or transition metal or the like in a number of points and is utilized in many applications while making the best use of its characteristic properties. For example, this oxide film is used as an oxide film in various electronic parts or devices, especially dielectric thin films used in capacitors or semiconductor devices, gate dielectric films of thin film transistor, reflectors of flat panel display, switching devices, or the like.
Such an oxide film used as a dielectric thin film of capacitor or semiconductor device, a gate dielectric film of thin film transistor, or the like is required to have such properties that it is thin, minute and free from a pinhole and that its surface is smooth (flat). Since an oxide film obtained by anodization of a material to be treated containing a valve metal as a principal component theoretically has such characteristic features that it does not form a pinhole at the time of film formation and that it is minute, it has hitherto been considered that such an oxide film is useful for these applications.
Various materials have hitherto been proposed as a forming electrolyte used in such anodization. For example, in JP-A-2000-328293, by using a forming electrolyte having an aromatic carboxylic acid salt dissolved in ethylene glycol and water as solvents, an oxide film having high dielectric properties and high hillock resistance is formed within a short period of time.
However, in recent years, following the microfabrication of various devices, there has hitherto been a demand to form an oxide film which is more minute and higher in the surface smoothness. Also, from the viewpoint of making it easy to treat a waste liquid while taking into consideration the environment, there has been a demand to reduce the amount of a non-aqueous solvent in the forming electrolyte and to increase the water content. Also, since ethylene glycol is subjective to “The Law concerning Reporting, etc. of Releases to the Environment of Specific Chemical Substances and Promoting Improvements in Their Management (Law for PRTR)”, it is preferable that the use of ethylene glycol is avoided, if possible.
In addition, the case where water is contained in a forming electrolyte involves a problem that the film quality of an oxide film to be formed varies depending upon a fluctuation of the water content in the forming electrolyte. Thus, there is also a demand to reduce influences by this fluctuation of the water content.
There have also been made various proposals with respect to electrical conditions of anodic oxidation in the anodization.
If an oxidation current density in anodic oxidation is increased, there may be a case where the growth of an oxide film is too fast so that a largely roughed film against the film thickness is liable to be formed, whereby an oxide film having a smooth surface is not formed. Then, in order to solve this problem the anodic oxidation is in general carried out through a two-stage anodic oxidation process including a constant current anodic oxidation step and a constant voltage anodic oxidation step. That is, this process is a measure in which anodic oxidation is first carried out at a constant current until the voltage reaches a value corresponding to an expected film thickness, thereby forming an oxide film; and thereafter, in order to restore the roughing of the formed oxide film, the voltage is kept at a constant voltage as it stands until the current is thoroughly reduced.
However, even in such a two-stage anodic oxidation process, if the current density of the constant current anodic oxidation step is excessively increased or the time of the constant voltage anodic oxidation step is too short, there is involved a problem that surface roughing of the formed oxide film is caused.
J-A-6-216389 describes that anodic oxidation by an alternating current containing a direct current component is carried out, thereby enhancing the film quality of a formed oxide film. However, this method involved a problem that since an alternating current is used, a special and expensive power source is necessary. Also, JP-A-9-138420 describes that constant current anodic oxidation is carried out at a very high current density, thereby obtaining a flat film free from waving. However, in such anodic oxidation at a high current density, even if waving is overcome, fine roughing cannot be avoided, and therefore, it is difficult to apply this method to a reflector or a device for which fine and high-degree surface smoothness is required.
Patent Document 1: JP-A-2000-328293
Patent Document 2: JP-A-6-216389
Patent Document 3: JP-A-9-138420